Ultrasonic-flow measuring devices are often applied in process and automation technology. They permit volume flow rate in a pipeline to be determined without contact and in simple manner.
Known ultrasonic flow measuring devices frequently work on the basis of the Doppler principle or on the basis of the travel-time difference principle.
In the case of the travel-time difference principle, the different travel times of ultrasonic pulses relative to the flow direction of the liquid are evaluated.
For this, ultrasonic pulses are sent both with as well as against the flow. From the travel-time difference, the flow velocity can be determined. In the case of a known diameter of the section of the pipeline, the volume flow rate can also be determined.
In the case of the Doppler-principle, ultrasonic waves with an ascertained frequency are coupled into the liquid, and the ultrasonic waves reflected by the liquid are evaluated. From the frequency shift between the coupled and reflected waves, the flow velocity of the liquid can likewise be determined.
Reflections in the liquid only occur, however, when small air bubbles or impurities are present, so that this principle is mainly applicable in the case of impure liquids.
The ultrasonic waves are produced or received with the assistance of so-called ultrasonic transducers. For this, ultrasonic transducers are firmly placed on the tube wall of the relevant pipeline section. More recently, clamp-on ultrasonic flow measuring systems are also obtainable. In the case of these systems, the ultrasonic transducers are pressed on the tube wall with only a clamp. Such systems are known e.g. from European Patent EP-B-686255, and U.S. Pat. No. 4,484,478 or U.S. Pat. No. 4,598,593.
A further ultrasonic flow measuring device which works on the travel-time difference principle is known from U.S. Pat. No. 5,052,230. The travel time is, in such case, ascertained by means of bursts in the form of short, sinusoidal, ultrasonic pulses.
The ultrasonic transducers normally are composed of a piezo element, also called piezo, for short, and a coupling element, also called a coupling wedge or, not so frequently, lead-in body, of synthetic material, or plastic. The ultrasonic waves are produced in the piezo element and conveyed via the coupling element to the tube wall and from there led into the liquid. Since the velocities of sound in liquids and synthetic materials, or plastics, are different, the ultrasonic waves are refracted in passing from one medium to another. The angle of refraction is ascertained in a first approximation by Snell's law. The angle of refraction is, thus, dependent on the ratio of the propagation velocities in the two media.
Between the piezoelectric element and the coupling element, an adapting, or matching, layer is arranged. The adapting, or matching, layer performs, in such case, the function of transmitting the ultrasonic signal and simultaneously the reduction of a reflection caused by different acoustic impedances at interfaces between two materials.
German Patents DE2537788A1 and DE3832947A1 describe an adapting, or matching, layer of a synthetic material filled with hollow glass spheres. This adapting, or matching, layer possesses, in such case, an impedance, which lies between the impedance of the piezoelectric element and the impedance of the measured medium. The adapting, or matching, layer possesses a thickness of a fourth of the characteristic wavelength λ of the ultrasonic signal radiated by the piezoelectric element.
The theoretical best value of the thickness of an adapting, or matching, layer for the maximum transmission is determined by the equation
      λ    4    +      n    *          λ      2      with n=1, 2, 3 . . . , thus the uneven numbered multiple of a fourth of the wavelength λ of the ultrasonic signal. Holding for the optimal matching of the impedance isZA=√{square root over (ZP*ZK)}with ZA being the impedance of the adapting, or matching, layer, ZP the impedance of the piezo and ZK the impedance of the coupling element. For an established impedances of about 30 MRayl for the piezo and about 3 MRayl for the coupling element, there results an impedance for the adapting, or matching, layer of 9 to 10 MRayl. Today, applied materials, such as glass with 11 to 14 MRayl or aluminum with about 17 Mrayl, lie relatively close to the calculated optimum.
In order for the material selection and the thickness of the adapting, or matching, layer to be less limited, a stack of many matching layers on top of one another is provided in German Patent DE4028315A1. The individual layers can have, in such case, thicknesses smaller than ¼ of the wavelength λT of the transducer and different impedances. In such case, however, no resonance matching is performed. On the rear-side of the piezo, thus on the side of the piezoelectric radial oscillator facing away from the matching layers, a damping body, also called backing, is placed supplementally and attenuates the oscillation on the rear-side of the piezo. In such case, however, oscillatory energy is withdrawn, which thus cannot be used for the measuring. The composite of radial oscillator and matching layers executes a thickness oscillation, whose eigenresonance is the mentioned wavelength of the transducer. The non-resonant matching of the impedance with the matching layers smaller than ¼ of the transducer wavelength has a negligible influence on the measuring of the first three half waves of the ultrasonic transducer operated in burst mode.
In U.S. Pat. No. 5,251,490, a stack of up to twelve matching layers is likewise shown, which, however, reaches in sum the thickness of λ/4. With this stack, a resonance matching, and thus a measuring, can again occur with a packet of many consecutive oscillations.
A plurality of matching layers stacked on top of one another is also disclosed in European Patent EP0305519A1. The matching layers of synthetic material, such as polyester, and metal, such as copper, are connected with one another via a usual adhesive connection. The thicknesses of the individual layers changes stepwise from the piezo to the measurement object. A so constructed ultrasonic transducer with an adapting, or matching, layer is directly placed on the object that is to be scanned by sound. Matching to a coupling element, or even to a pipeline, is not provided.